Flexible electrostatographic imaging members are well known in the art. Typical flexible electrostatographic imaging members include, for example: (1) electrophotographic imaging member belts (photoreceptors) commonly utilized in electrophotographic (xerographic) processing systems; (2) electroreceptors such as ionographic imaging member belts for electrographic imaging systems; and (3) intermediate toner image transfer members such as an intermediate toner image transferring belt which is used to remove the toner images from a photoreceptor surface and then transfer the very images onto a receiving paper. The flexible electrostatographic imaging members may be seamless or seamed belts. Typical electrophotographic imaging member belts include a charge transport layer and a charge generating layer on one side of a supporting substrate layer and an anti-curl back coating coated onto the opposite side of the substrate layer. A typical electrographic imaging member belt does, however, have a more simple material structure; it includes a dielectric imaging layer on one side of a supporting substrate and an anti-curl back coating on the opposite side of the substrate. Although the scope of the present invention covers the preparation of all types of flexible electrostatographic imaging members, for reason of simplicity, the discussion hereinafter will focus only on flexible electrophotographic imaging members.
Electrophotographic flexible imaging members may include a photoconductive layer including a single layer or composite layers. Since typical electrophotographic imaging members exhibit undesirable upward imaging member curling, an anti-curl back coating is required to offset the curl. Thus, the application of anti-curl back coating (“ACBC”) is necessary to effect the appropriate imaging member flatness.
One type of composite photoconductive layer used in electrophotography is illustrated in U.S. Pat. No. 4,265,990, the entire disclosure of which is incorporated by reference herein, which describes a photosensitive member having at least two electrically operative layers. One layer includes a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer with the photoconductive layer sandwiched between the contiguous charge transport layer and the conductive layer, the outer surface of the charge transport layer is normally charged with a uniform charge of a negative polarity and the supporting electrode is utilized as an anode. Obviously, the supporting electrode may still function as an anode when the charge transport layer is sandwiched between the supporting electrode and the photoconductive layer. The charge transport layer in this latter embodiment must be capable of supporting the injection of photogenerated electrons from the photoconductive layer and transporting the electrons through the charge transport layer. Photosensitive members having at least two electrically operative layers, as disclosed above, provide excellent electrostatic latent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. The resulting toner image is usually transferred to a suitable receiving member such as paper.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during extended cycling. Moreover, complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements, including narrow operating limits, on photoreceptors. For flexible electrophotographic imaging members having a belt configuration, the numerous layers found in modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a belt in negatively charging electrophotographic imaging systems includes a substrate support, a conductive layer, a hole blocking layer, an adhesive layer, a charge generating layer, a charge transport layer, and a conductive ground strip layer adjacent to one edge of the imaging layers. This photoreceptor belt also includes additional layers such as an anti-curl back coating to achieve the desired photoreceptor belt flatness.
In a machine service environment, a flexible imaging member belt, mounted on a belt supporting module, is generally exposed to repetitive electrophotographic image cycling which subjects the outer-most charge transport layer to mechanical fatigue as the imaging member belt bends and flexes over the belt drive roller and all other belt module support rollers, as well as sliding bend contact of the anti-curl back coating over all the belt module support rollers and against each backer bar's curving surface. This repetitive imaging member belt cycling leads to a gradual deterioration in the physical/mechanical integrity of the exposed outer anti-curl back coating below the belt and the charge transport layer on the top of the belt, leading to excessive anti-curl back coating wear, as well as causing premature onset of fatigue charge transport layer cracking. Anti-curl back coating wear results in a dirty machine environment with debris being deposited over the backer bar's surface to create high protrusion spots. These high protrusion spots then poke the back side of the imaging member belt during cyclic belt motion to further exacerbate and hasten the early onset of dynamic fatigue charge transport layer cracking. The cracks developed in the charge transport layer are found to manifest themselves into copy printout defects, which thereby adversely affect the image quality on the receiving paper. In essence, the appearance of charge transport cracking cuts short the imaging member belt's intended functional life.
When a production web stock of several thousand feet of coated multilayered photoreceptor is obtained after finishing the charge transport layer coating/drying process, it is seen to spontaneously curl upward toward the applied coating layers, and, therefore, requires an anti-curl back coating to be applied to the backside of the substrate support (i.e., opposite to the side having the charge transport layer) to offset the curl and render the desired photoreceptor web stock flatness. The exhibition of spontaneous upward photoreceptor web stock curling after completion of charge transport layer coating has been determined to be the consequence of thermal contraction mismatch between the applied charge transport layer and the substrate support under the conditions of elevated temperature heating/drying the solution applied wet coating and eventual cooling down to room ambient temperature. Since the charge transport layer in a typical prior art photoreceptor device has a coefficient of thermal contraction approximately 3½ times larger than that of the substrate support, it does, upon cooling down to room ambient temperature, result in greater dimensional contraction than that of the substrate support, causing upward photoreceptor curling which then requires the anti-curl back coating to balance the curling effect and provide photoreceptor flatness.
Seamed flexible photoreceptor belts are fabricated from rectangular sheets cut from an electrophotographic imaging member web stock having an anti-curl backing layer. The cut sheets are generally rectangular in shape. All edges may be of the same length or one pair of parallel edges may be longer than the other pair of parallel edges. The sheet is formed into a belt by joining the overlapping opposite marginal end regions of the sheet. A seam is typically produced in the overlapping opposite marginal end regions at the point of joining. Joining may be effected by any suitable means such as welding (including ultrasonic processes), gluing, taping, pressure/heat fusing, and the like. However, ultrasonic seam welding is generally the preferred method of joining because it is rapid, clean (no application of solvents) and produces a thin and narrow seam. The ultrasonic seam welding process involves a mechanical pounding action of a welding horn which generates a sufficient amount of heat energy at the contiguous overlapping marginal end regions of the imaging member sheet to maximize melting of one or more layers therein. A typical ultrasonic welding process is carried out by holding down the overlapping ends of the flexible imaging member sheet with vacuum onto a flat anvil and guiding the flat end of the ultrasonic vibrating horn transversely across the width of the sheet and directly over the overlapped junction to form a welded seam having excellent seam rupture strength and good belt flatness. A seamed flexible photoreceptor belt having good overall physical flatness, rendered by utilizing an anti-curl back coating, is of crucial importance; otherwise, under a dynamic belt functioning condition for a belt without the anti-curl back coating, upward belt curling will have significant surface distance variance to the machine charging devices causing non-uniform charge density dispensing on the photoreceptor belt surface to degrade copy printout quality. Moreover, photoreceptor belt upward curling will also physically interact/interfere with the xerographic subsystems, particularly, for example, in those machines employing a hybrid scavengeless development (HSD) or hybrid jumping development (HJD) subsystem, leading to undesirable artifacts which then manifest into printout defects in the final image copies.
Several earlier prior art references have disclosed successful fabrication of electrostatographic imaging members utilizing particularly selected support substrates for imaging member structural simplification without the need for an anti-curl back coating to render imaging member flatness. The particular substrates selected, such as polyether sulfone, polyvinyl fluoride, Makrofol, special formulated polyimide, amorphous polyethylene terephthalate, and the like, could provide curl-free imaging member devices, nevertheless the advantage gained by elimination of the anti-curl back coating was outweighed by the generation of some undesirable outcomes. For example, polyether sulfone and Makrofol used as an imaging member substrate support are susceptible to attack by the solvent used for the imaging layer coating solution, while polyvinyl chloride and amorphous polyethylene terephthalate both have low glass transition temperature (Tg) unable to withstand the high temperature exposure of the drying process for imaging member coatings of each layer, which during imaging member manufacturing, is elevated to about 130° C. The special polyimide substrate support is too expensive to justify its value for imaging member production implementation. In other words, the attempts to eliminate the need for an anti-curl back coating have been overcome by the creation of another set of undesirable problems.
With all the undesirables associated with the prior art mentioned above, it becomes clear that fabrication of flexible seamed photoreceptor belts having an anti-curl back coating is of crucial importance to render a photoreceptor belt with physical flatness for good machine functioning result as well as to eliminate the undesirable copy printout defects. The innovative formulation of an anti-curl back coating is necessary to have robust wear resistant properties to provide a clean imaging member belt machine function environment. Further, the anti-curl back coating formulation should be low cost so as to cut unit imaging member belt manufacturing cost.
U.S. Pat. No. 5,089,369, the entire disclosure of which is incorporated by reference herein, describes an electrophotographic imaging member having a supporting substrate and a charge generating layer, where the supporting substrate is made of a material having a thermal contraction coefficient which is substantially the same as that of the charge generating layer. Substrate materials having a thermal contraction coefficient value between about 5.0×10−5/° C. and about 9.0×10−5/° C. are preferred for use in combination with a benzimidazole perylene charge generating layer to resolve charge generation cracking problems and eliminate the need of an anti-curl back coating.
U.S. Pat. No. 5,167,987, the entire disclosure of which is incorporated by reference herein, describes a process for fabricating an electrostatographic imaging member including providing a flexible substrate including a solid thermoplastic polymer, forming an imaging layer coating including a film forming polymer on the substrate, heating the coating and substrate, cooling the coating and substrate, and applying sufficient predetermined biaxial tensions to the substrate while the imaging layer coating and substrate are at a temperature greater than the glass transition temperature of the imaging layer coating to substantially compensate for all dimensional thermal contraction mismatches between the substrate and the imaging layer coating during cooling of the imaging layer coating and the substrate, removing application of the biaxial tensions to the substrate, and cooling the substrate whereby the final hardened and cooled imaging layer coating and substrate are substantially free of internal stress and strain to yield a resulting curl-free imaging member without the need of an anti-curl back coating.
U.S. Pat. No. 4,983,481, the entire disclosure of which is incorporated by reference herein, describes an imaging member having improved resistance to curling without an anti-curl backing coating. The imaging member includes a flexible supporting substrate layer, an electrically conductive layer, an optional adhesive layer, a charge generating layer, and a charge transport layer, the supporting substrate layer having a thermal contraction coefficient substantially identical to the thermal contraction coefficient of the charge transport. The supporting substrate may be a flexible biaxially oriented layer.
While the above mentioned flexible imaging members may be suitable for their intended purposes to resolve specific problems and improve imaging member function, resolution of these problems have often created new ones. For example, supporting substrates such as polyether sulfone and Makrofol, which have thermal contraction coefficients closely matching that of the coated charge transport layer, are effective in keeping the electrophotographic imaging member from curling upward curling, but are oftentimes susceptible to attack and can be damaged by solvents used for the charge transport layer coating solution, thereby rendering the imaging member useless. Other prior art substrate supports, though having good thermal contraction coefficient matching properties such as Tedlar or Melinar, and thus yield curl-free electrophotographic imaging member without the need for an anti-curl back coating, inherently have low glass transition temperatures, Tg, and have been determined to be insufficient for imaging member fabrication. Moreover, another prior art disclosure of application of a biaxial tensioning stress onto imaging member, which was maintained at an elevated temperature slightly above the Tg of the charge transport layer to render imaging member flatness, was found to involve a cumbersome batch process, which process was deemed to be very costly to implement for imaging member production adaptation.
Since imaging member belt fabrication with total elimination of an anti-curl back coating still remains to be a conceptual idea that has been experimentally demonstrated in laboratory scale, it is yet far from being a production implemented manufacturing reality. Therefore, there is a continued need to improve the fabrication of imaging members, particularly providing it with a robust and low cost anti-curl back coating in the multilayered electrophotographic imaging member belt design that enhances the belt's machine mechanical performance.